TUESDAY, May 4 (HealthDay News) -- Almost one-third of U.S. parents surveyed delayed vaccines for their very young children in 2008, and 12% simply refused to have their children immunized, possibly making them more vulnerable to illness, a new study shows.

Physicians report that children who don't receive recommended vaccine doses by the time they're 2 years old are at risk of developing a variety of diseases. But some anti-vaccine activists contend that the shots can cause side effects, including autism, although health officials say repeated studies have failed to uncover such a link.

For this study, researchers analyzed the results of a 2008 national survey of parents and health-care providers.

Thirty-one percent of parents with children aged 24 to 35 months reported that they'd delayed vaccine doses on purpose in 2008. In fact, the percentage of parents who either delayed or refused to immunize their children grew from 22% in 2003 to 39% in 2008.

Not all the non-vaccinating parents oppose immunization; 44% of the parents who didn't vaccinate their children on schedule said their child was ill. In addition, 27% thought too many shots were recommended; 26% questioned whether vaccines were effective; 25% were concerned about autism; and 24% said they feared side effects or thought vaccines weren't entirely safe.

The study was scheduled to be presented Tuesday at the Pediatric Academic Societies' annual meeting in Vancouver, British Columbia, Canada.